MDEV-6109: EXPLAIN JSON

- Add first testcases
- Don't overquote when printing conditions
- Other small output fixes

mysql-test/r/explain_json.result

@@ -0,0 +1,41 @@
+drop table if exists t0,t1;
+create table t0(a int);
+insert into t0 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+explain format=json select * from t0;
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "table": {
+      "table_name": "t0",
+      "access_type": "ALL",
+      "rows": 10,
+      "filtered": 100
+    }
+  }
+}
+explain format=json select * from t0 where 1>2;
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "table": {
+      "message": "Impossible WHERE"
+    }
+  }
+}
+explain format=json select * from t0 where a<3;
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "table": {
+      "table_name": "t0",
+      "access_type": "ALL",
+      "rows": 10,
+      "filtered": 100,
+      "attached_condition": "(t0.a < 3)"
+    }
+  }
+}
+drop table t0;

mysql-test/t/explain_json.test

@@ -0,0 +1,27 @@
+#
+# EXPLAIN FORMAT=JSON tests. These are tests developed for MariaDB.
+#
+--disable_warnings
+drop table if exists t0,t1;
+--enable_warnings
+
+create table t0(a int);
+insert into t0 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+
+explain format=json select * from t0;
+
+explain format=json select * from t0 where 1>2;
+
+explain format=json select * from t0 where a<3;
+
+#create table t1 (a int, b int, filler char(32), key(a));
+#insert into t1 
+#select 
+#  A.a + B.a* 10 + C.a * 100, 
+#  A.a + B.a* 10 + C.a * 100,
+#  'filler'
+#from t0 A, t0 B, t0 C;
+#
+#explain format=json select * from t0,t1 where t1.a=t0.a;
+
+drop table t0;